You can do this by removing Responsible signer Access for the parent who will not be signing the contract, deleting the contract entirely, and generating a new contract that only requires one signature. If the parents have already paid, manually enter that payment has already been received on the new contract.

To edit the Responsible signer access, do the following:

1. Core.
2. People finder > search for your parent
3. Select Contact card tab
5. Under Relationships click Manage Relationships.
7. Update the parental access fields by Deselecting the "Responsible signer" box.
8. Click Save.

To delete the Contract and add a new one, do the following:

1. Enrollment management 
2. People Finder > search for your candidate
3. Click on the Contracts Tab
4. Click on the trashcan icon to delete.
5. Select Add New.
6.Select the appropriate contract type.
7. Select the school year for which the contract should be generated. The selected contract type can only be assigned to each candidate/student one time for each
8. Enter additional information about the contract, including tuition, fee, and publication information.
9. Select Save.